A surface coil of this kind is known from EP-A 280 908 which corresponds to U.S. Pat. No. 4,924,868. Therein, the transverse conductors are connected to the longitudinal conductors via electronic switches so that the effective surface of such a coil depends on which switches are closed. One of the two longitudinal conductors therein forms a part of a coupling device for connecting the surface coil to an RF transmitter or an RF receiver.
The known surface coil is suitable for generating RF magnetic fields which extend perpendicularly to the surface defined by the longitudinal and transverse conductors and for receiving such fields.
During magnetic resonance examinations, the patient to be examined is exposed to a strong, steady, uniform magnetic field. The magnet for this steady magnetic field is generally shaped as a hollow cylinder in which the patient to be examined is introduced so that the longitudinal axis of the patient extends parallel to or is coincident with the axis of the hollow cylinder.
For the excitation of spin resonance phenomena and for the reception of spin resonance signals it is necessary that the magnetic fields (to be generated or received) extend perpendicular to the steady magnetic field. In general this condition can be suitably satisfied by means of the known surface coil when it is arranged on the surface of the body of the patient and when the uniform, steady magnetic field extends in the direction of the cylinder axis as is customary thus far. This is because the RF magnetic field generated by the surface coil then extends perpendicular to the surface of the patient and hence also perpendicular to the direction of the axis of the hollow cylinder and the direction of the steady magnetic field.
However, recently magnets have appeared which also have the shape of a hollow-cylinder but which generate a steady magnetic field which extends perpendicular to the axis of the hollow cylinder and hence perpendicular to the longitudinal axis of the patient. When the known surface coil is used in such a magnet, the condition of orthogonality between the steady field and the RF magnetic field can no longer be satisfied in all cases.